Division of Ecological and Water Resources (Waters)

Groundwater pollution sensitivity

Definition of pollution sensitivity

The Minnesota Department of Natural Resources (DNR) defines a sensitive area as a geographic area characterized by natural features where there is significant risk of groundwater degradation from activities conducted at or near the land surface (MS § 103H.101). The DNR has developed criteria and guidelines to assess sensitive areas to encourage a consistent approach to assessing geologic sensitivity in Minnesota (Geologic Sensitivity Workgroup, 1991). Assessments are based on the geologic and hydrogeologic factors that affect the ability of geologic materials to restrict the downward migration of contaminants to the groundwater of interest. This approach is called geologic sensitivity.

At the DNR, two types of geologic sensitivity are considered: the sensitivity of the near-surface materials, and the sensitivity of buried aquifers. The sensitivity of the near-surface materials is based on estimated travel times through the unsaturated, or vadose, zone from land surface to the water table. The sensitivity of buried aquifers is considered separately.

Geologic sensitivity assessments are typically done on a county scale, typically 1:100,000. A countywide assessment of pollution sensitivity requires some generalizing assumptions. One assumption is that flow paths from the land surface through the soil and underlying sediments to an aquifer are vertical. Horizontal flow paths may be important in specific instances, but they have not been adequately mapped and are not considered when calculating the pollution sensitivity of the near-surface materials. However, the permeability of deeper sediments is evaluated only qualitatively when calculating the pollution sensitivity of buried sand and gravel aquifers and the top of the bedrock.

Groundwater travel time through the near-surface materials are based on estimated transmission rates for unsaturated soils and surficial geologic units.

The assessment of the geologic sensitivity to pollution for the near-surface materials estimates the time required for water to travel from the land surface to a depth of 10 feet (Procedure for determining near-surface pollution sensitivity maps, v.1, DNR, 2014).The near-surface materials sensitivity assessment was developed by estimating transmission rates through soils and surficial geologic units based on the Natural Resources Conservation Service (NRCS) hydrologic rating for soils (NRCS, 2009) and the geologic unit texture of deeper parent materials, from surficial geologic maps in the county atlas series.

Groundwater travel time to buried sand and gravel aquifers and sensitivity rating

Groundwater sensitivity to pollution is best understood in relation to travel time, which is the approximate time that elapses from when a drop of water infiltrates the land surface until it enters an aquifer or reaches a specific target such as a spring. This is also often called residence time. Several techniques can be used to estimate the travel time of groundwater in an aquifer, including use of dye traces, radioactive and stable isotopes, and chlorofluorocarbons. Radiometric dating using radioactive isotopes of carbon (carbon-14) and hydrogen (tritium) are commonly used to estimate groundwater residence time (Alexander and Alexander, 1989). Estimated or measured travel times are inversely related to sensitivity: shorter travel times may indicate higher sensitivity and longer travel times may indicate lower sensitivity.

DNR Waters has defined five relative classes of geologic sensitivity (Very High, High, Medium, Low, and Very Low) that are based on overlapping time of travel ranges. The pollution sensitivity of an aquifer is assumed to be inversely proportional to the time of travel. In addition, contaminants are assumed to travel at the same rate as water. Very High sensitivity indicates that water moving downward from the surface may reach the groundwater system within hours to months. In these areas, there is little time to respond to and prevent aquifer contamination. Conversely, Low sensitivity indicates there is time for a surface contamination source to be investigated, and possibly corrected, before serious groundwater pollution develops.

Time of Travel Criteria

Geologic sensitivity ratings are based on the time required for water at or near the surface to travel vertically to the water table or other groundwater of interest. Longer travel times imply a lower sensitivity to pollution. Dye trace, tritium, and carbon-14 studies can indicate the relative ages of groundwater.

Relatively high sensitivity does not mean that water quality has been or will be degraded. If there are no contaminant sources, for example, pollution will not occur. Also, relatively low sensitivity does not guarantee that groundwater is or will remain uncontaminated. For instance, leakage from an unsealed well may bypass the natural protection of geologic materials, allowing contaminated water from one aquifer to directly enter another aquifer.

Applying the DNR pollution sensitivity criteria and guidelines

The DNR Waters Criteria and Guidelines report describes the process for preparing maps that show areas of relative sensitivity representing known or estimated subsurface conditions. The maps are intended for use as screening tools and guides to indicate where additional information might be desirable to support land use or resource protection decisions. The Criteria and Guidelines report discusses three types or "levels" of geologic sensitivity maps: Level 1 assessment - preliminary; Level 2 assessment - vadose zone materials; and Level 3 assessment - deeper aquifers. The three levels of pollution sensitivity assessment provide procedures to assess the geologic sensitivity of the water table as well as deeper aquifers. Selection of an assessment level depends on the ground water of interest and the available information to conduct an assessment.

Maps can be prepared manually as described in the Criteria and Guidelines report. More commonly, digital files describing the distribution of geologic and hydrogeologic factors are digitally combined using geographic information system (GIS) technology. A table of sensitivity ratings based on the factors and estimated travel times is developed and applied to the combined factors using GIS, generating the pollution sensitivity map. A stack diagram schematic shows this process in general.

A pollution sensitivity map gives a broad perspective; more detailed geologic and hydrogeologic information must be evaluated for a site-specific investigation. The sensitivity evaluation is based on the assumption that contaminants travel at the same rate as infiltrating water; therefore, the assessment does not consider any specific contaminant. Contaminants are assumed to originate at or near the land surface and move downward; lateral movement is ignored. Within these limitations, however, the map can serve as a screening tool to estimate the potential impacts of certain activities and land uses on ground-water quality at a general scale.

Additional resources

In addition to the examples included in the DNR Waters Criteria and Guidelines report, a study by the University of Minnesota evaluated the sensitivity level assessment methods for Scott County, Minnesota. The Pilot Study examined the original three assessment levels and several variations and applied four methods to check and validate the methodology.

The DNR, in cooperation with the Minnesota Geological Survey, develops a geologic sensitivity assessment as part of a comprehensive report that includes both geologic and groundwater conditions. For links to completed reports in the County Geologic Atlas Series and the Regional Hydrogeologic Assessment Series please see the map of study areas.

Most studies to date have focused on a county or regional area. However, a statewide evaluation of groundwater contamination susceptibility is available, prepared by the Minnesota Pollution Control Agency in 1989. Developed using GIS technology, the digital files are distributed by the Land Management Information Center (Minnesota Department of Administration) through their GeoGateway search service. The digital data is also on their EPPL7/EPIC 2000 dataset CD-ROM.

Geologic Sensitivity Workgroup, 1991, Criteria and guidelines for assessing geologic sensitivity of ground water resources in Minnesota: Minnesota Department of Natural Resources, Division of Waters, St. Paul, MN, 122 p. Original printed document scanned December 2003. A few pages contain minor clarifications of the original text.